1. Field of the Invention
The invention relates to a position indication system for an element, such as a rod, which is movable in a direction along its longitudinal axis, and more specifically to a method for temperature compensating a position indication system which determines the relative position of a control rod within the core of a nuclear reactor.
2. Description of the Prior Art
There are a great number of applications requiring remote monitoring of the position of an elongated movable metallic member having one degree of freedom along its longitudinal axis. For example, in the nuclear art it is necessary to raise and lower control rods within the reactor core for controlling the energy output of the nuclear reactor. The use of the term "control rod" is used herein to include any member positioned within the reactor which alters the reactivity of the reactor. Thus, this includes rods which serve other purposes besides normal control use. The sue of the word "rod" is synonymous with "control rod" for the purposes of this invention.
The control rods are located within proximity of nuclear fuel elements comprising nuclear fissionable material. Generally, the greater the number of neutrons within the core of the reactor, the greater the number of fissions of the fuel atoms that take place, and consequently, the greater the amount of energy released. Energy, in the form of heat, is removed from the reactive region by a coolant which flows through the region and then flows to a heat exchanger wherein the heat from the reactor coolant is used to generate steam for driving turbines to transform heat energy into electrical energy. To decrease the energy output of the nuclear reactor, the control rods, made of materials which absorb neutrons, are inserted within the reactive region, commonly known as the nuclear core. The greater the number of control rods and the further the control rods are inserted within the reactive region, the greater the number of neutrons that will be absorbed and hence the energy output of the reactor will decrease. Conversely, to increase the energy output of the reactor, the nuclear control rods are withdrawn from the reactive region. Consequently, the number of neutrons absorbed decreases, the number of fissions increase, and the energy output of the reactor increases.
For pressurized water reactors, it is of the utmost importance to know the accurate position of each of the control rods. Differences of over 15 inches between neighboring rods adversely affect fuel management. In addition, knowledge of the position of the rods versus thermal power output gives an indication of the condition of the reactor, thus, the degree of fuel burnup. Therefore, extremely reliable control rod drive and position monitoring systems must be employed in order to maintain the safe and reliable operating status of the reactor.
One system presently employed to lower and raise the control rods utilizes a jack-type electromechanical mechanism which employs a plurality of electrical coils to incrementally insert or withdraw each control rod within the reactor. Such a system is more fully described in U.S. Pat. No. 3,158,766 issued to E. Frisch and assigned to the assignee of the present invention.
In a pressurized water reactor three mechanisms are generally available for providing an indication of rod position; a step counter, a movable in-core flux-mapping system and a rod position indication system.
The step counter provides an indirect measurement by electrically counting the number of mechanical steps commanded by the rod control system. As an indirect measurement system, it cannot detect mechanical malfunctions that prevent rod movement when commanded.
The movable in-core flux-mapping system does provide direct measurement of control rod position as it is moved in a thimble close to the rod of interest. However, it is used for this purpose only as a backup system because of the mechanical wear and operator interaction problems associated with its continual use. Therefore, the rod position indication system is the primary means of direct measurement of control rod position.
As previously described, the control rods move within a pressure vessel and are attached to drive rods, which can be incrementally moved in a forward or reverse direction by a drive mechanism, such as the magnetic jack mechanism described in the cited Frisch patent. The drive rod extends longitudinally through the pressure vessel, along the axis of movement of the control rod, into the sealed, pressurized environment of the rod travel housing. Since it is of the utmost importance to maintain the sealed integrity of the pressurized vessel, mechanical penetrations are kept at a minimum to reduce the likelihood of loss of the pressurized environment contained therein. Accordingly, no mechanical penetrations are permitted for detecting the relative position of the control rods within the core of the reactor. Inasmuch as it would be a very difficult task to detect the position of the actual control rods, it has been the practice to detect the position of the drive rods which are affixedly coupled thereto and translate drive rod position into control rod location within the core of the reactor vessel.
A number of detectors have been used in the past to determine drive rod position. In one such detector a permanent magnet is located on top of the drive rod for movement therewith inside the rod travel housing. A series of reed switches and associated precision resistors are disposed outside the rod travel housing along its entire length, the resistors being connected in electrical series with each other. Movement of the drive rod, and hence the magnet, activates the reed switches as the magnetic flux of the magnet comes within range of the respective switches. The activation of a reed switch shorts out the associated precision resistor. A measurement of the impedance of the series connected resistors thus provides an indication of the rod position. A drawback of this detector is that a significant amount of magnetic flux is generated by the mechanism that lifts the rod, which totally disrupts the reed switches, giving rise to an erroneous reading of position indication. Also, the field of action of the magnetic flux of the magnet is not particularly well contained which could inadvertently actuate several adjacent switches simultaneously, which again leads to an erroneous reading.
In another known detector a permanent magnet is employed at the top of the drive rod as discussed above, however the reed switches and precision resistors are replaced with a conductive wire that is tightly strung along the side of the travel housing. A large current pulse is passed through the wire which causes the wire to twist under the action of a magnetomotive force at the zone at which the flux of the magnet passes through the wire. The twisting action propagates up the wire and induces a voltage pulse in a transducer located at the end of the wire. The time delay from the initiation of the current pulse in the wire to the occurrence of a voltage pulse by the transducer corresponds to the position of the magnet, and thus of the control rod. This detector has several disadvantages. Complicated mechanical means are required to damp the twisting action of the wire to prevent continuous oscillation thereof up and down the wire. In addition since the voltage pulse generated by the transducer is relatively small, it must be amplified by an immediate set of electronics which by necessity is located in a hostile environment of extremely high temperatures and radiation fields, leading to a high rate of failure of the detector electronics.
In a further known detector, a long single winding extends along the length of the rod travel housing and rod position is measured as a function of impedance changes in the winding as the position of drive rod changes within the rod travel housing. The impedance contributions of winding resistance, however, is not entirely predictable thus reducing the reliability of this type of detector.
An analog detector is known which avoids many of the above drawbacks. This detector includes a plurality of layered, wound coils concentrically arranged in a stack and supported by a nonmagnetic stainless steel tubular substructure that is slid over the nonmagnetic rod travel housing. The coils are arranged alternately as primary and secondary coils, with all the primary coils connected in series and all the secondary coils connected in series. The coils form, in effect, a long linear voltage transformer distributed over the height of the travel housing such that the coupling from primary to secondary is affected by the extent to which the magnetic drive rod penetrates the coil stack. Rod position is determined by applying a constant sinusoidal excitation current to the primary and measuring the voltage induced across the secondary. The magnitude of the induced secondary voltage corresponds to rod position.
The primary advantages that the transformer type of detector provides over the other detectors are: (1) there is no requirement for a permanent magnet on top of the drive rod and within the primary coolant; (2) there is no active circuitry required within the hostile environments of either the containment building or reactor head area; and (3) when the primary is excited by a precision current source and the secondary voltage is sensed with a high input impedance so that little current actually flows through the secondary, then the less predictable contributions of winding resistance and leakage inductance can be ignored. The transformer type of detector, however, has an accuracy problem in that the secondary voltage drifts with changes in the operating conditions of the reactor. A primary source of this drift has been traced to changes in the permeability and resistivity of the drive rod with variations in drive rod temperature. This problem requires frequent recalibration which is both tedious and time consuming and often resulting in delayed operation of the reactor.
U.S. Pat. Nos. 3,846,771 and 3,893,090, each describe a detector employing digital techniques which is more accurate than any of the foregoing detectors and avoids many of the above drawbacks. The digital detector, however, is expensive, which is a particularly important factor when consideration is given to repairing or improving an existing detector vis-a-vis total replacement thereof. For example, in a situation where the transformer type detector has been installed, it would be preferable if a relatively inexpensive means could be devised to compensate for drift, rather than replacing the detector and related electronics with the more expensive digital detector and its related electronics. In this way, the above noted advantages of the transformer type of detector are preserved and the expense of complete replacement is saved.